1. Field of the Invention
The present invention relates to electrical drive systems. In particular, the invention relates to electrical drive systems incorporating variable reluctance electric motors. The invention also relates to variable reluctance motors per se and in particular to two-phase constructions of such motors.
2. Description of the Prior Art
In a variable reluctance machine, high permeability ferromagnetic stationary and moving parts having pole surfaces separated by the smallest airgap consistent with mechanical clearance may be arranged to form magnetic paths of low reluctance except for highly saturated constriction zones determined by the overlap between the poles, so that the magnetic flux is determined primarily by the position of the moving part and as little as possible by the intensity of the excitation current or currents. The theory underlying the functioning of variable reluctance machines is discussed in the following papers: "Tangential forces in overlapped pole geometries incorporating ideally saturable material", John V. Byrne, IEEE Transactions on Magnetics, Vol. Mag-8, No. 1, March 1972; "Saturable overlapping rectangular poles", John V. Byrne and William J. O'Connor, IEEE Annals No. 509MA923-3, 1975; and "Magnetic forces in idealised saturable-pole configurations", William J. O'Connor, IEE Proc., Vol. 127, Pt.B, No. 1, Jan. 1980.
In a multi-phase construction of a variable reluctance motor, such as is described in U.S. Pat. No. 3,956,678 of Byrne et al, this constriction zone may be located at or adjacent to the overlapping poleface surfaces of one or both of the relatively displaceable parts of the machine by dimensioning the machine so that the cross-sectional area of the ferromagnetic material in the flux path at the variable interface in the poleface region is less than the cross-section available to magnetic flux elsewhere in the flux path, throughout the working stroke. The working stroke is determined by the extent of the relative mechanical displacement of the stationary and moving parts during which a substantially uniform rate of flux increase prevails, i.e. the displacement increment during which the cross-sectional area of the flux path at the constriction zone continues to increase, and it terminates when the increase in the cross-sectional area of the flux path ceases. In such multi-phase constructions of a reluctance motor, the area of the stator pole face is generally less than or substantially equal to the cross-section available for flux at the waist of the pole, i.e. the region of the pole extending circumferentially between the winding spaces in a rotational machine construction and joining the stator poleface region to the stator yoke. In some arrangements, the stator poleface area may somewhat exceed the waist area, but the constriction may remain defined at the overlap by means of, for example, rotor pole skewing, so that the area available for the flux path in the rotor pole remains less than the waist area of the stator pole, right up to full pole overlap. Thus, mechanical limitations on the length of the working stroke are imposed by the various design constraints of reluctance machines, even in the case of variants such as those incorporating rotor skewing. In three or four phase machines, these constraints need not be of major significance, in that the proliferation of phases means that another phase is always ready to take over rotor drive as the working stroke of a particular pair of cooperating rotor and stator poles comes to an end. Thus torque continuity is assured and the machine may be started in either direction from any rotor position. This is not, however, the case for a reluctance motor having less than three phases, and while self-start may be achieved by extending the arcuate extent of the rotor poles, as described in U.S. Pat. No. 3,956,678 of Byrne et al, the resulting configuration is limited to continuous torque in a single preferred direction of rotation.
While three and four phase reluctance motors are of their nature self-starting, the control circuit complexity and cost necessitated by the use of three phases generally shows no clear cost advantage over a comparable induction motor system. Neither is there usually any reduction in the total kVA rating of the power semiconductor devices needed for the reluctance motor control circuit. In the case of four-phase constructions of reluctance motors, the control circuit complexity and cost are greater again than the requirements for the three-phase configuration.
In said U.S. Pat. No. 3,956,678 of Byrne et al, a two-phase reluctance motor system is described, in which a working stroke of 90.degree. per phase is achieved by making each rotor poleface arc 100.degree., while retaining a relatively conventional stator poleface arc of 50.degree.. Thus the angular extent of the rotor poleface surface is approximately twice that of the stator poleface. In order to provide the required flux constriction zone at the variable interface, approximately one-half of the rotor poleface surface is underlaid or backed by an iron depletion region, this region being defined by trapezoidal slots in the arrangement particularly described in the specification. This configuration enables the required linear increase of flux with rotation to continue over virtually the full extent of pole relative displacement until the rotor pole portion of full or undepleted iron density comes substsantially into full alignment with the stator pole, magnetic saturation being confined to the neighborhood of the mechanically variable interface or overlap between the stator and rotor poles. The region of depleted iron density thus forms a leading edge region of the rotor pole during normal operation of the motor described. As this region begins to overlap the stator pole, flux starts to increase in linear dependence on rotor angular displacement. The depleted region is sized so that when it comes into full alignment with the stator pole, the flux level is approximately one-half of its maximum value. Flux then continues to build up linearly towards its maximum as further rotation of the rotor brings the rotor pole portion of full or undepleted iron density into substantially full overlap with the stator pole. This machine is self-starting in a direction corresponding to said regions of depleted iron density being leading edge regions of the rotor poles during rotor rotation. The self-starting facility is achieved by the asymmetrical extension of the rotor poles necessary for the exploitation of saturation in the machine.
More recently, a paper by J. C. Compter, of Philips Research Laboratories, Eindhoven, The Netherlands, entitlted "Microprocessor-Controlled Single-Phase Reluctance Motor", described a single-phase reluctance machine suitable for high speed applications in the power range up to approximately one horsepower. Such motors may be used in, for example, vacuum cleaners and portable tools. In these applications, the reluctance machine may exhibit clear advantages over the motors conventionally used in such products, for example, as series commutator motors. In the arrangement described by Compter, the rotor is held in a position favorable for starting by a small permanent magnet. Start-up and speed control are microprocessor-controlled. The costs entailed by this aspect of the system are claimed to be justified by virtue of the simplification of the power circuit, this requiring just one main switching device.